Nanocarrier-based drug delivery systems (DDSs) offer new opportunities to improve bioactivity and prolong bioavailability of drugs for cancer therapy1-4. To further enhance biological specificity and therapeutic efficacy, significant efforts have been devoted to explore active targeting5 and stimuli responsive DDSs6, which specifically accumulate at the tumor site and are stimulated to release drugs within tumor cells. These stimuli can be a variety of external signals, such as temperature7, light8, magnetic field9, ultrasound10 and electric current11 as well as physiological factors, such as pH12, redox potential13, enzymatic activities14 and glucose levels15, 16. A number of nanosystems including liposomes17, polymeric nanoparticles18 and inorganic particles19 have been developed to apply these triggering cues to tailor pharmacokinetics and enhance delivery efficiency. For example, acid-sensitive species have been widely incorporated in intracellular delivery vehicles that undergo an endocytosis pathway, thereby allowing drugs to be readily released in acidic compartments12. The gradient of reduced glutathione (GSH) enables polymeric nanogels crosslinked with GSH-cleavable disulfides to efficiently release payloads inside cells20. The proteases highly expressed in specific tumor microenvironments facilitate nanocarriers consisting of relevant peptidyl substrates to release drugs on demand14.